Continuous liquid flow system

ABSTRACT

A system for continuous delivery of a measured flow of liquid to a processing apparatus includes a reservoir for holding a bulk supply of the liquid and an intermediate vessel. A first pump delivers liquid from the intermediate vessel to processing equipment at one rate. A second pump delivers liquid from the reservoir to the intermediate vessel at a second much faster rate. A monitor device continuously monitors the weight of the vessel and controls the first pump accordingly to deliver the required weight of liquid at the required rate to the processing equipment. The monitoring device determines when the weight of liquid in the intermediate vessel has reached a minimum position to initiate operation of the second pump to refill to vessel and a further device terminates operation of the second pump when the liquid level in the vessel reaches a maximum position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of continuously measuring theflow of a liquid. Pumping liquids often results in the generation ofbubbles which make it difficult to measure and control with conventionalflow measuring devices.

2. The Relevant Technology

In the field of gaseous phase sterilisation the most common method togenerate the vapour is by evaporating an aqueous solution of thesterilant on a hot surface. Typically, the solution would be 35% w/whydrogen peroxide. Such solutions tend to be unstable and give offbubbles of gas, which interfere with the conventional flow measuringsystems.

This problem was recognised and dealt with in EP O 662 844 B1, bydrawing the sterilising solution from a container into an accumulatorand measuring the weight loss in the container. Whilst this techniqueallows for a known weight of liquid to be delivered to the evaporatorand then to turn this weight of liquid into a vapour it has twoshortcomings. Firstly, it is necessary to decide at the start of theprocess how much liquid will be required, and secondly it limits theamount of liquid that may be dispensed. The apparatus described in EPO662 844 B1 requires that the liquid flow rate to the evaporator isgreater than the flow rate used to fill the accumulator. This differencein the flow rate means that the accumulator cannot be refilled from thecontainer as the flow from the accumulator will always be greater thanthe flow with the accumulator.

It is also important in gaseous sterilisation process to be able tocontrol the concentration of the gas being delivered to the chamber tobe sterilised. The concentration will depend on the mass flow of thecarrier gas, normally air, and the rate at which liquid is evaporatedinto the air stream. The present invention not only deals with the firsttwo difficulties found in Patent EPO 662 844 B1 but also provides amethod of measuring and controlling the liquid flow to the evaporator.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method forcontinuously delivering a measured flow of liquid to a processingapparatus such as an evaporator when the liquid may be unstable andbubbles may be generated spontaneously in the liquid flow path.

The invention provides a system for continuous delivery of a measuredflow of liquid to a processing apparatus comprising a reservoir forholding a bulk supply of the liquid, an intermediate vessel, first meansto pump liquid from the intermediate vessel to processing equipment atone rate, second means to pump liquid from the reservoir to theintermediate vessel of a second much faster rate, means to monitorcontinuously the weight of the vessel and to determine the mass flowrate of liquid form the intermediate vessel and to control the firstpump means accordingly to deliver the required weight of liquid at therequired rate to the processing equipment, means to determine when theweight of liquid in the intermediate level has reached a minimumposition to initiate operation of the second pump means to refill thevessel and means to terminate operation of the second pump means whenthe liquid level in the vessel reaches a maximum position.

In one specific embodiment according to the invention, the systemconsists of a primary liquid reservoir, a measuring tube, and pumps totransfer the liquid from the reservoir to the measuring tube, and fromthe measuring tube to the liquid evaporator. A further pump may beprovided to remove any residual liquid from the measuring tube to thereservoir at the end of the sterilisation cycle. It is essential thatthe delivery rate of the pump feeding the liquid from the reservoir tothe measuring tube should be about 20 times faster than the maximumspeed of the pump delivering the liquid from the measuring tube to theevaporator. The liquid flow rate required to be delivered from themeasuring tube to the evaporator will depend on the mass flow of thecarrier gas and the required concentration of the sterilising gas.

The liquid is first pumped from the reservoir into the measuring tube.The pressure at the bottom of the measuring tube will increase with theheight of the column of liquid and is measured using a pressuretransducer.

When sufficient liquid has been delivered to the measuring tube the pumpdelivering the liquid will be stopped and the system is ready to startdelivering liquid to the evaporator.

As soon as it is required to start the sterilisation process liquid ispumped from the measuring tube to the evaporator, and the rate of changein the height of the column is measured using the pressure transducer.The change of pressure may be converted into a mass flow rate fromknowledge of the diameter of the measuring tube. Once this rate of flowis known it may be used to adjust the pump speed to correct anydeviations from the selected mass flow. Because this is a genuine massflow technique it eliminates the effects of bubble formation.

When the level of the liquid in the measuring tube falls to a pre-setlow level the filling pump is started and the measuring tube is refilledas before. During the refilling process the delivery pump to theevaporator runs at the last adjusted speed and the mass flow of theliquid in the evaporator is assumed to remain constant.

At the end of the sterilisation process the control system calculatesthe total mass of liquid delivered to the evaporator from the mass ofthe liquid delivered from the measuring tube, plus the calculated massflow during the time taken to re-fill the measuring tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a description of some specific embodiments of theinvention, reference being made to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a liquid delivery system according tothe invention; and

FIG. 2 is a diagram showing a modified arrangement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIG. 1 of the drawings, there is shown an apparatusfor producing a measured continuous flow of liquid such as water to aprocessing apparatus such as an evaporator.

The apparatus consists of a container 10 fluidly connected by a pipe 11to a column form measuring tube 12 via a pump 13. At the base of themeasuring tube is fitted a pressure transducer 14 to measure thepressure exerted by the column of liquid in the measuring tube. A Pump15 delivers liquid to an evaporator (not shown) from the measuring tube.An overflow tube 16 is provided to connect the top of the measuring tubeto the container to return excess fluid to the container should a faultarise and the measuring tube become overfilled. A pump 17 is used topump excess liquid at the end of the sterilisation process via pipe 18,which fluidly connects the base of the measuring tube to the container.

A control system, typically based on a Programmable Logic Controller(PLC), is used to control the operation of the apparatus. The pump 15 isa variable speed pump typically a peristaltic pump, and may be used tovary the liquid flow rate from the measuring tube to the evaporator. Thepump 13 is a fixed speed pump, typically having a mass flow rate about20 times greater than the maximum delivery rate of pump 15. Pump 17 isused to empty the measuring tube at the end of the cycle and may haveany convenient rate. Typically pump 13 and pump 17 are peristalticpumps, but any other type that is compatible with the liquid would besuitable. The liquid to be evaporated as the sterilizing agent is firstpumped from the container through the pipe fluidly connecting thecontainer to the measuring tube by the pump 13, until the required massof liquid as measured by the Pressure Transducer has been transferred.

Once the measuring tube is full then the system is ready to startdelivering the liquid to the evaporator using the pump 15. The initialspeed of the pump 15 is set by the control system to give the desiredmass flow rate. The initial speed of the pump is set by reference to thedata stored in the PLC from the calibration of the pump speed and theflow rate.

Once liquid is delivered to the evaporator by pump 13, the liquid levelin the measuring tube will start to fall, and hence the static pressuremeasured by the pressure transducer will also fall. By monitoring therate of fall of pressure the PLC is able to calculate the actualdelivery rate achieved by pump 15. The PLC is then able to adjust thespeed of pump 15 to adjust for any deviation of the actual mass flowfrom the required mass flow.

Should the sterilisation not be completed when the liquid in themeasuring tube falls to the minimum level, then the pump 13 will bestarted by the PLC filling the measuring tube. During the period of timethat it takes for the pump 13 to fill the measuring tube the pump 15remains running at the last adjusted speed. As soon as the measuringtube is re-filled then automatic adjustment of the speed of pump 15 isresumed to maintain the required mass flow rate.

The number of times that the measuring tube may be re-filled is onlylimited to the amount of liquid available in the container. Should verylong sterilisation processes be required it is possible to replenish theliquid in the container either manually or with a separate automaticsystem, which senses the level of liquid in container using a dip tube.

At the end of the sterilisation period the pump 17 returns any remainingliquid from the measuring tube to the container. The overflow pipe whichfluidly connects the top of the measuring tube to the container isprovided in the event of an equipment failure and pump 13 shouldcontinue to operate after the measuring tube has been filled.

Further safety features may be programmed into the PLC to ensure thatthe tube is filled within a fixed period of time, to avoid the problemof a failure of pump 13 or having an empty container.

The total mass flow may be calculated from the change in pressure in themeasuring tube, and if necessary adjusted for the liquid delivered whilethe measuring tube is being re-filled. This adjustment may be made fromknowledge of the time taken to re-fill the tube and the flow rateimmediately before the re-filling was started.

The measuring tube may be re-filled any number of times and each timethis occurs an adjustment must be added to the total mass flow.

The system is particularly suitable for use in the sterilizing apparatusdescribed and illustrated in UK Patent Application No. 9922364.6.

An alternative arrangement is shown in FIG. 2, in which the measuringtube and a pressure transducer are replaced by a reservoir 20 placed onan electronic weigh scale 21. In this configuration the process isidentical except that the signal from the weigh scale replaces thesignal from the pressure transducer, and it is essential that the pipesthat supply and extract liquid from the reservoir do not disturb theelectronic balance. The overflow pipe may be removed and replaced by adip contact 22 to ensure that the reservoir 20 is not overfilled.

1. A system for continuous delivery of a measured flow of liquid to aprocessing apparatus, the system comprising: a reservoir for holding abulk supply of the liquid; an intermediate vessel; a first pump forsupplying liquid from the intermediate vessel to processing equipment ata first rate; a second pump for supplying liquid from the reservoir tothe intermediate vessel at a second rate, the second rate being fasterthan the first rate; and a controller comprising: a sensor adapted todetermine when the weight of liquid in the intermediate vessel hasreached a minimum position to initiate operation of the second pump torefill the vessel and adapted to terminate operation of the second pumpwhen the weight of liquid in the intermediate vessel has reached amaximum position; and a monitor adapted to continuously monitor theweight of liquid in the intermediate vessel to determine a mass flowrate of liquid from the intermediate vessel and adapted to adjust thefirst pump while the liquid in the intermediate vessel falls from themaximum position to the minimum position such that the mass flow rate ofliquid from the intermediate vessel substantially matches a requiredflow rate of the processing equipment.
 2. A system as claimed in claim1, wherein the sensor comprises a pressure transducer adjacent theintermediate vessel for determining the weight of any liquid therein. 3.A system as claimed in claim 1, wherein the controller for initiatingoperation of the second pump to recharge the intermediate vessel fromthe reservoir deactivates the control of the first pump in accordancewith the mass flow rate of liquid from the intermediate vessel, and thefirst pump runs at a constant rate during the period of replenishment ofthe intermediate vessel.
 4. A system as claimed in claim 1, furthercomprising a third pump configured to return any unused liquid in theintermediate vessel to the reservoir.
 5. A system for continuousdelivery of a measured flow of liquid to a processing apparatuscomprising: a reservoir for holding a bulk supply of the liquid, anintermediate vessel, first means for pumping liquid from theintermediate vessel to processing equipment at one rate, second meansfor pumping liquid from the reservoir to the intermediate vessel at asecond faster rate, and means for controlling a mass flow rate of liquidpumped from the intermediate vessel, the means for controllingcomprising: means for determining when the weight of liquid in theintermediate vessel has reached a minimum position to initiate operationof the second means for pumping to refill the vessel; means forterminating operation of the second means for pumping when the liquidlevel in the vessel reaches a maximum position; and means for monitoringcontinuously the weight of liquid in the intermediate vessel and fordetermining the mass flow rate of liquid from the intermediate vesseland for adjusting the first means for pumping while the liquid in theintermediate vessel falls from the maximum position to the minimumposition such that the mass flow rate of the liquid from theintermediate vessel substantially matches a required flow rate of theprocessing equipment.
 6. The system in claim 5, wherein the intermediatevessel has a pressure transducer positioned adjacent the bottom of theintermediate vessel for determining the weight of any liquid therein. 7.A system as claimed in claim 5, wherein the means for controlling forinitiating operation of the second means for pumping to recharge theintermediate vessel from the reservoir deactivates the control of thefirst means for pumping in accordance with the mass flow rate of liquidfrom the intermediate vessel, the first means for pumping running at aconstant rate during the period of replenishment of the intermediatevessel.
 8. A system as claimed in claim 5, wherein a further pumparrangement is provided to return any unused liquid in the intermediatevessel to the reservoir.